Separation approaches towards understanding supramolecular aggregate formation of humic acid

Abstract

Humic acid (HA) is classified as a large humic substance (HS) with a broad molecular weight distribution of 102– 106 Da which is soluble in alkaline solutions and insoluble in acidic solutions. HA plays an important role for the transport, bioavailability and the fate of various pollutants. This includes the many heavy metal ions which strongly complex with HA in environment. In the last two decades, it has been hypothesized that HAs exist as supramolecular aggregates of small organic compounds of molecular weight of around 600–1000 [1]. However, the mass distribution of HA units and the mechanism of supramolecular formation remain controversial. The supramolecular formation of HA is no doubt driven through intermolecular forces among HA unit molecules, such as bridging by metal ion coordination, hydrogen bonding, and hydrophobic interaction. However, obtaining experimental evidence of those interactions at a molecular level is difficult. The chemical interpretation of HA supramolecules is important for developing functional humic acids [2–4], as well as to fully understand their impact on the environment. This short article is a review of the types of separation techniques combined with various spectrometric methods employed in the analysis of HA aggregation in recently reported studies in the period from 2019 to 2021.